1. Field of the Invention
This invention relates to an ophthalmological instrument having an anterior portion observation optical system for observing the anterior portion of an eye to be tested.
2. Discussion of the Related Art
A conventional ophthalmological instrument, for example, a noncontact type tonometer, having an anterior portion observation system for observing the anterior portion of an eye to be tested is known. This anterior portion observation system is adapted to make it easy to perform the positional adjustment or the so-called alignment verification of the instrument with respect to the eye to be tested.
In this type of the instrument, an optical system for observing the anterior portion of the eye to be tested is provided so that an image of the anterior portion of the eye to be tested is formed on a predetermined imaging plane when the distance between the eye to be tested and the instrument is proper. Owing to the foregoing, in order to perform the alignment verification, the instrument may be in a position close enough to the eye to be tested so that the image of the anterior portion of the eye to be tested can be observed approximately. For this purpose, after the instrument is gradually brought closer to the eye into eye to be tested first in order to bring into the focus, an optical axis is aligned and the distance is correctly adjusted.
In this conventional instrument, for example, in a non-contact type tonometer, a proper working distance between a nozzle for ejecting air and the cornea of the eye to be tested is approximately 11 mm. The anterior portion of the eye is not seen until the instrument approaches this distance and therefore, the adjustment is performed by feel until the instrument is brought close to the eye so that the anterior portion can be seen to some extent. This adjustment requires much time and professional skill.